Automotive-type internal combustion engine exhaust gas emission control system

ABSTRACT

A timing circuit is connected between a threshold switch providing alternately voltages representative of excess, or absence of oxygen in the exhaust emission from the engine, the timing circuit measuring the time interval between change-over of the threshold switch as determined by changes in the composition of the exhaust emission; if the change-over is within a predetermined timing range, a control switch is held in one position; if the change-over rate drops, however, indicative of malfunction in the exhaust emission sensor, or associated connected equipment, the switch is placed into another state whereupon it will command a fuel-air control amplifier to provide a control ratio of a predetermined value and, if desired, additionally provide a sensed output to control an alarm or other trouble indicator. Switching change of the control switch can be overriden by other signals available within the automotive vehicle.

United States Patent [191 Schmidt 51 Nov. 4, 1975 Peter-Jiirgen Schmidt,Schwieberdingen, Germany [73] Assignee: Robert Bosch GmbH,

Gerlingen-Schillerhohe, Germany [22] Filed: Dec. 13, 1973 [21] Appl.No.: 424,464

[75] lnventor:

[51] Int. Cl. F02B 3/00 [58] Field of Search l23/32 EA [56] ReferencesCited UNITED STATES PATENTS l/l974 Schmidt 123/32 EA PrimaryExaminerCharles J. Myhre Assistant ExaminerJoseph A. Cangelosi Attorney,Agent, or Firm-Flynn & Frishauf ABSTRACT A timing circuit is connectedbetween a threshold switch providing alternately voltages representativeof excess, or absence of oxygen in the exhaust emission from the engine,the timing circuit measuring the time interval between change-over ofthe threshold switch as determined by changes in the composition of theexhaust emission; if the change-over is within a predetermined timingrange, a control switch is held in one position; if the change-over ratedrops, however, indicative of malfunction in the exhaust emissionsensor, or associated connected equipment, the switch is placed intoanother state whereupon it will command a fuel-air control amplifier toprovide a control ratio of a predetermined value and, if desired,additionally provide a sensed output to control an alarm or othertrouble indicator. Switching change of the control switch can beoverriden by other signals available within the automotive vehicle.

11 Claims, 2 Drawing Figures U.S. Patent Nov. 4; 1975 AUTOMOTIVE-TYPEINTERNAL COMBUSTION ENGINE EXHAUST GAS EMISSION CONTROL SYSTEM Thepresent invention relates to a system to decrease the noxious componentsin the emission from internal combustion engines, and more particularlyto such a system in which the mass ratio of fuel and air of the mixtureapplied to the internal combustion engine is controlled by means of anintegrating controller which, in turn, has input signals applied theretorepresentative of compliance with, or deviation from sensed standards inthe exhaust from the engine.

Exhaust sensors located to be responsive to the composition of theexhaust gases from an internal combustion engine, and providingelectrical output signals, have previously been proposed. To control amass ratio of air-fuel being applied to the internal combustion engine,also called A control, fuel and air, in suitable relationships, areapplied by means of a carburetor or an electronically controlled fuelinjection system to the engine. The control system maintains the massratio of fuel and air being supplied at such a value that the exhaustfrom the engine has a minimum of noxious components. This is the case ifthe exhaust is just slightly reducing, that is, if the air-fuel mixturebeing applied .is maintained at a value which is just belowstoichiometric ratio. At this just slightly under stoichiometric value,this exhaust gases will have a composition which will have a minimum ofnoxious components, or can be limited to a minimum of noxiouscomponents, if the exhaust system of the internal combustion engine hasa catalytic reactor to treat the exhaust gases.

The exhaust gas sensor may fail. This sensor, located in gas-sensingrelationship to the exhaust gases of the engine, typically in theexhaust system thereof, will then no longer provide an output signal.Failure of an output signal from a typical sensor is, electrically, theequivalent of an indication that the air-fuel mixture is lean. A controlsystem connected thereto will then control the input to the engine tocompensate for this erroneous lean mixture and command a rich fuelairmixture being applied to the engine.

It is an object of the present invention to provide a control system inwhich failure of the sensing device, or at least a portion of thecontrol circuitry will provide an output indication to the operator bycommanding an alarm system, for example, and/or changing the operatingconditions of the internal combustion engine to such an extent that theoperator of the engine, typically of a vehicle, cannot but fail tonotice, and thus is forced to repair, or have repaired the vehicle andparticularly the exhaust system, for example by replacing the sensor.

Control systems in which exhaust sensors are used and also employingintegrating amplifiers have output signals derived from the sensorswhich jump between extreme values, indicative of presence or absence ofoxygen in the exhaust system. The integrating amplifier then controlsthe system in such a manner that, on the average, the oxygen beingsensed is a minimum. The system includes a threshold switch, which isresponsive to changes in output signals from the sensor.

SUBJECT MATTER OF THE PRESENT INVENTION Briefly, a timing circuit isconnected to the threshold switch, and controls a switch which is heldin a certain position when the threshold switch has a change-overrepetition rate indicative of normal or ordinary operation, but permits,or causes change-over of the switch if there is trouble, or malfunctionin the sensor, or the circuit connected thereto. The timing switch, thensensing a different repetition rate of change-over of the thresholdswitch, commands change of the switching position which, in turn,provides an output signal which can be directly applied to the controlcircuit of the integrating amplifier, to an alarm or warning device, orto other apparatus to influence the operation of the internal combustionengine.

The invention will be described by way of example with reference to theaccompanying drawings, wherein:

FIG. 1 is a general, highly schematic circuit diagram and illustratingan example of a A-control circuit and supervisory circuit; and

FIG. 2 is a fragmentary portion of another embodiment of the circuit ofFIG. 1.

An oxygen sensor 1 is located in sensing relation to the exhaust gasesof an internal combustion engine (not shown). The output sensor 1provides an electrical signal which is a characteristic of thecomposition of the gases within the exhaust from the internal combustionengine. The oxygen sensor is, preferably, enclosed in a shield,schematically indicated at l. The sensor, as well as the controlcircuit, are powered from supply buses 3, 10, connected, for example, tothe battery circuit of an internal combustion engine, or any othersuitable d-c circuit which may be subject to voltage fluctuations.

The electrical signal from sensor 1 is applied to the base of atransistor 2 connected as an emitter follower. The collector oftransistor 2 is directly connected to bus 3; the emitter of thetransistor 2 is connected over a resistor 4 to a junction J1. Thecollector-emitter path of transistor 2 and the series connected resistor4 form one leg of a first branch of a bridge 5. A second leg of thebranch ofi-the bridge 5 is formed by the series circuit of thecollector-emitter path of a second transistor 6 and a resistor 7. Thebase of transistor 6 is connected to a voltage divider formed byresistors 8, 9, connected over a common resistor 12 between supply buses10 and 3. Resistor 9 is adjustable. The two resistors 8, 9 have a Zenerdiode 11 connected in parallel thereto.

The third and fourth legs of the bridge 5, forming the second branch,are formed by a resistor 13, connected to the common bus 3 and aresistor 14 connected to the common bus '10, both being connected tojunction .12, which forms, together with junction J l, the diagonalconnection of the bridge.

The diagonal of the bridge is connected to an operational amplifier I5,junction J2 being connected over coupling resistor 17 with thenon-inverting input, and junction J1 being connected over couplingresistor 16 to the inverting input of the operational amplifier 15. Anoutput resistor 18 is connected to the output of the operationalamplifier l5, and connected to the common supply 10. The output of theoperational amplifier 15 is further connected to a junction point of twobase resistors 19, 20 for transistors 21, 22, respectively. Transistors21, 22 have their emitters connected to respective tap points of twoserially connected voltage dividers formed of resistors 23, 24, 26,respectively. The transistors 21, 22 vary the transfer of the voltageswing derived from operational amplifier l5, and thus can change thevoltage difference of the output signal being derived from theoperational amplifier 15 and further connected to an integratingamplifier. The emitter of transistor 21 is connected to the junctionbetween resistors 23, 24; the emitter of transistor 22 is connected tothe junction between resistors 25, 26. The collectors of the twotransistors 21, 22 are connected together. The jointly connectedcollectors of transistors 21, 22 are connected over a transfer switch 27with an input or coupling resistor 28 of an operational amplifier 29,which functions as an integrator, having an integrating capacitor 30connected between its output and its inverting input, to which thecoupling resistor 28 is likewise connected. Switch 27 can selectivelyconnect resistor 31, 32 as coupling resistors, rather than the resistor28 to the inverting input of operational amplifier 29. The non-invertinginput of operational amplifier 29 is connected over acoupling resistor33 with the junction of resistors 24, 25. The output of the operationalamplifier 29 is connected over a resistor 34 to the common supply busand, further, to an output terminal 29'. The output terminal 29 can beused to control, if necessary over an additional power amplifier, theairfuel mixture of an internal combustion engine, for example byproviding a position output which influences supply of fuel, or air tothe internal combustion engine, or by modifying the injection period offuel injection valves in the fuel injection system.

Operational amplifier is further connected by line 15 to a timingcircuit 36. The output line 15' is connected to a monostable flip-flop(FF) which triggers the timing circuit 36. The monostable FF includes atransistor 37, coupled over a coupling capacitor 39 and a diode 38, thecathode of diode 38 being connected to the base of transistor 37. Aresistor 40' connects to the common supply bus 3. The junction point ofthe anode of the diode 38 and capacitor 39 is connected to a resistor 41which is connected to the common supply bus 10. The emitter oftransistor 37 is connected to the common supply bus 3; the collector oftransistor 37 is connected over resistor 42 to supply bus 10; a diode 43connects from the collector of transistor 37 to one terminal of acapacitor 44, the second terminal of which is connected to supply bus 3.Resistor 45, of compara- I tively high value, is connected in parallelto the capacitor 44. A coupling resistor 46 connects from the commonconnection of capacitor 44, diode 43 and resistor 45 to the base of aninput transistor 47 of a control change-over switch 46. Transistor 47 isconnected as an emitter-follower, and has its collector connected to bus10 and its emitter to bus 3, if necessary over an emitter-resistor (notshown). The emitter of transistor 47 is further connected over aresistor 48 to the base of a switching transistor 49. A resistor 50connects the base to common line 3. The junction between resistor 48,resistor 50, and base 49 has one or more resistors 51, 51 connectedthereto, which, in turn, are connected to respective terminals 52, 52'.Further resistors similar to resistors 51, 51 and having furtherterminals similar to terminals 52, 52, may be connected to the samejunction, and therefore the connection to resistor 51' and terminal 52is shown in dashed lines. The terminals 52, 52 may have .variouselectrical signals applied thereto corresponding to operating oroperation parameters of the internal combustion engine, and

4 to influence the state of the switching transistor 49, as will appearin detail below. .The emitter of the switching transistor 49 isconnected to supply bus 3. The collector of transistor 49 is connectedto an output terminal 49; further, the collector is connected over aresistor 53 to common bus 10. A connection line, including a diode 54,connects from the collector of transistor 49, and hence output terminal49', to the common line connecting the collectors of transistors 21, 22,and further forming the common switch terminal of switch 27.

OPERATION The output sensing signal from sensor 1 is connected into thefirst branch of the bridge 5 over transistor 2. This electrical signal,which is representative of the composition of the exhaust gases of theinternal combustion engine, forms the actual value for the controlcircuit. The sensors 1 have a high internal resistance when thetemperature in the exhaust gases is still low. Thus, the control circuitmust have high input resistance and, therefore, transistor 2 isoperatedas an emitter-follower and, together with its load resistor 4, providesthe necessary input resistance for the circuit.

The command value, that is, the value which the air number A shouldhave, and to which the air-fuel mixture is to. be controlled so that theexhaust gases from the engine will have a predetermined composition, isformed by means of Zener diode 11. The resulting signal is applied overthe voltage divider formed of resistors 8, 9 and transistor 6 into thesecond arm of the branch of bridge 5. The command value can be adjustedby adjusting resistor 9. Transistors 2 and 6, in the first branch of thebridge, are of opposite conductivity type, and so connected that theyare temperature-compensating.

The comparison circuit for the command value and actual value isconnected as a bridge, and thus is essentially independent of variationsin supply voltage, supplied over the common lines 3, 10. The bridge isin balance, that is, command and actual value are equal, if thecollector-emitter paths of transistors 2, 6 have the same voltagethereacross. This is the case if the output signal from sensor 1, andthe commanded value which the signal should have are equal. If thecommand value and actual value deviate, a voltage will appear across thediagonal terminals J1, J2 of the bridge. Let it be assumed that theoutput voltage of the sensing signal from sensor 1, that is, the actualvalue is greater than the commanded value. The inverting input to theoperational amplifier 15 will have a higher voltage applied than at thenon-inverting input. Operational amplifier 15, functioning as acomparator circuit, provides an output signal which will beapproximately at the voltage of the supply line 3. In the opposite case,that is if the actual value is smaller than the commanded value, theoutput signal of operational amplifier 15 will jump to a voltage whichcorresponds approximately to that of supply line 10.

The voltage swing or voltage jump at the input to integrator 29, 30 isdecreased by an intermediately connected circuit 40, which includestransistors 21, 22. The voltage jump is decreased and, therefore, thevalue of integrating capacitor 30 can be decreased. The output signal ofoperational amplifier 15 controls either the one or the other oftransistors 21, 22 to be in conductive condition, the other, or the onetransistor, then, being blocked. A signal will be applied over therespectively conductive transistor to the input network of theintegrator 29, 30 which has a smaller voltage swing than that of theoutput signal of the operational amplifier.

Switch 27 is provided in order to change the time constant of theintegrator 29, 30. The output signal of the integrator 29, 30 thencontrols a suitable control element in such a manner that the air-fuelmixture applied to the internal combustion engine is so changed that itcorresponds to a predetermined commanded ratio, resulting in exhaust ofminimum noxious components. Switch 27 is shown schematically; asreferred to in US. Pat. Nos. 3,782,347 and 3,831,564, both assigned tothe assignee of this application, the actual construction of the switchcan be different. The switch 27 can be a controlled semiconductorswitch, for example transistors in series with resistors correspondingto schematically indicated resistors 28, 31, 32. The transistors areselectively rendered conductive in accordance with desired integrationtime constants, as explained in the cross-referenced applications.

Let it be assumed that the sensor 1 and the connected circuit arefunctioning properly. Operational amplifier 15 will change over, thatis, will change state in relatively short time intervals, the outputvoltage of the operational amplifier 15 switching between the voltage ofsupply line and supply line 3, periodically. The output of theoperational amplifier thus provides, in short intervals, positive andnegative pulses. Upon each negative jump, monostable FF, that istransistor 37, is triggered by transfer of a pulse over capacitor 39,that is, transistor 37 is blocked by a period of time determined bycapacitor 39. When transistor 37 blocks, resistor 42 and diode 43 willhave no current flowing thereover, and capacitor 44 can charge. Thetime, determined by capacitor 39, is so selected that the capacitor 44can be fully charged. After this time has elapsed, and when transistor37 again becomes conductive, capacitor 44 can discharge over thehigh-resistance resistor 45 and the input of transistor 47, operated asan emitter-follower. The discharge time of the storage capacitor 44 isso selected that, assuming proper operation of the control system and ofthe sensor 1, a residual charge will always remain in capacitor 44 andcapacitor 44 will never completely discharge, but always retain acertain minimum charge. Thus, the switch 46, formed by transistors 47,49, is not changed over. Switching transistor 49 is held in conductivestate by transistor Let it be assumed that sensor 1 fails, or that thecontrol system fails, that is, that no output signal will be availableat the output of operational amplifier 15. Capacitor 44 can nowdischarge completely. This blocks transistor 49. The air-fuel mixturewhich is applied to the internal combustion engine is then controlled insuch a manner, by output from integrating amplifier 29, 30, that themixture will go to the lean direction. This is obtained by transferringa signal over diode 54 which applies its signal to the resistors 28 (or31, 32, respectively, if connected) in such a direction that theintegrator 29, 30 will integrate in the direction to command a leanmixture. The operating performance of the internal combustion enginewill become worse, forcing the operator of the engine, usually a motorvehicle, to en gage in maintenance and repair, that is, typically to replace sensor 1.

The output signal available at the collector of transistor 49, and hencethe terminal 49', can be used in other ways than to connect a signalover diode 54, as shown.

It may be applied, for example, at other points in a fuelair mixturesupply system, or elsewhere in the vehicle, for example by interferingwith proper electrical operation of the electrical components of thesystem, in order to inhibit normal operation of the engine and toprovide an indication to the operator. The output signal can also beused to control a warning device, which informs the driver that thesensor 1, or a portion of the control system, has failed.

Referring to FIG. 2, the resistor 53 can be omitted and the circuit,otherwise identical to that shown in FIG. 1, can be broken at terminalsX-X'. Instead of resistor 53, the coil 53 of a relay is connectedbetween terminals X-X, which, when energized, pulls in a relay contact60 to energize an alarm 61, for example an alarm lamp, connected withthe switch 60 between buses 3 and 10. Alarm system 61 can be used inlieu of, or in combination with diode 54, or to interfere in any otherway with the operation of the motor vehicle electrical system, or, bycontrol of other parameters, with any other element, in addition toproviding a warning signal.

The monostable FF is controlled by the negative going flank, due to thepresence of diode 38, and uses a single transistor. It can be replacedby the well-known monostable FF circuits with two transistors, so thatthe positive as well as the negative switching flank from the output ofthe operationalamplifier 15 will trigger the timing circuit to chargecapacitor 44 twice as often. The size of capacitor 44 can thus bedecreased, at the cost of an additional transistor.

The control systems to influence the operation of the internalcombustion engine, that is, to control the airfuel mixture, should notbe effective under some operating conditions of the internal combustionengine. Some of such typical conditions are: when the exhaust sensor 1is still cold, that is, upon starting, upon warmup of the internalcombustion engine; or upon full-load operation of the engine; or uponidling operation of the engine. In all these instances, electricalsignals can be applied over terminals 52, 52' to the junction ofresistors 48, 50, over appropriate coupling resistors 51, 51' whichelectrical signals are characteristic of the one or the other operatingcondition under which the transistor 49 is to be held in conductivecondition, so that the control circuit and a warning device (FIG. 2), ifused, is disabled; in other words, the normal operation of transistor 49is overriden, so that no output signal will be available from transistor49 at terminal 49'. The operating conditions of the internal combustionengines will, under such override conditions, not be affected, since,under those operating conditions, the control system is not operatingbut, due to the special conditions then pertaining, normal operation ofthe internal combustion engine, without malfunction or troubleindication, is desired.

Various changes and modifications may be made in accordance with theinventive concept.

I claim:

1. In an internal combustion engine exhaust emission control systemhaving means (1) sensing the composition of the exhaust gases andproviding a sensed electrical signal,

an integrating control amplifier (29, 30) connected to said sensingmeans (1) and providing an output signal (29) which is available tocommand the relative mass ratio (A) of air-fuel applied to the engme;

and a threshold switch connecting the sensing means (1) to theintegrating control amplifier and operative to change switching stateabove, and below its threshold level, said threshold switch, upon normaloperation of the system, cyclically switching between its states,

timing circuit (36) connected to the threshold switch (15) and measuringthe time intervals between change-over of switching state of thethreshold switch (15), as determined by changing output from the sensingmeans (1) the timing circuit sensing the intervals between change-overof the threshold switch (15); and

a change-over switch (46; 47, 49) controlled by said timing circuit (36)the timing circuit controlling said change-over switch to be in a firststate when the duration of the intervals between change-over of thethreshold switch are below a predetermined timing interval, indicativeof proper operation of the sensing means and of the threshold switch,the timing circuit controlling said change-over switch (46; 47, 49) tobe in another state when the switching interval since the lastchange-over of the threshold switch is above said predeterminedinterval, said change-over switch, when in the second state, providingan output signal (49) indicative of excess time lapse betweenchange-over of the threshold switch (15) and representative of systemmalfunction.

2. System according to claim 1, further comprising a monostableflip-flop (37, 39) connected between the threshold switch (15) and thetiming circuit (36) and triggered into unstable, state by change ofstate of the threshold switch (15);

and wherein said timing circuit comprises a capacitor (44) connected tobe charged during one state of the monostable flip-flop (37, 39) and adischarge circuit (45) permitting discharge of the capacitor during theother state of the monostable flip-flop, said discharge circuit having atime constant which prevents complete discharge of the capacitor whenthe switching interval of the threshold switch (15) is within saidpredetermined value to provide a residual charge on the capacitor (44),said residual charge holding said change-over switch (46) in said stateindicative of proper operation.

3. System according to claim 2, wherein the capacitor (44) is chargedduring the unstable state of the monostable flip-flop (37, 39) anddischarged during the stable state thereof and said discharge circuithas a high time constant and is connected to and controlling 8 saidchange-over switch (46) to change state indicative of trouble operationupon discharge of the capacitor below a predetermined level.

4. System according to claim 1, wherein the integrating controlamplifier comprises an operational amplifier (29);

and a coupling circuit (54) is provided between the output (49') of thechange-over switch (46) and the inverting input of the operationalamplifier (29) to modify the input signal applied thereto.

5. System according to claim 4, wherein the coupling circuit comprises adiode (54).

6. System according to claim 4, further comprising selectively variableinput resistance means (28, 31, 32) connected to the coupling networkand the operational amplifier (29, 30).

7. System according to claim 2, wherein the monostable flip-flop (37,39) comprises a transistor (37) having its emitter-collector pathconnected in parallel to the capacitor (44) of the timing circuit;

and a coupling capacitor (39) and a diode (38) coupling networkconnecting the output of the threshold switch (15) to the controlelectrode of the transistor (37).

8. System according to claim 1, wherein the timing circuit comprises acapacitor (44);

and the change-over switch (46) comprises an emitter-follower inputtransistor (47) connected to the timing capacitor (44), and a switchingoutput transistor (49) connected to and controlled by said inputtransistor (47).

9. System according to claim 1, further comprising alarm circuit means(53 60, 61) connected to the output (49') of the change-over switch (46)to provide an alarm output signal upon sensing of trouble operation.

10. System according to claim 1, further comprising at least oneoverride connection terminal (52, 52') connected to the change-overswitch (46) to hold said switch in a position indicating properoperation of the sensing means to override control of the integratingcontrol amplifier (29, 30) by said change-over switch (46) uponenergization of at least one of said override terminals.

1 1. System according to claim 1, wherein the relative mass ratio (A) ofthe air-fuel applied to the engine is controlled to apply a lean mixtureto the engine when the timing circuit has sensed a time interval fromthe last change-over of the threshold switch (15) which is above thetime duration of the predetermined interval.

1. In an internal combustion engine exhaust emission control systemhaving means (1) sensing the composition of the exhaust gases andproviding a sensed electrical signal, an integrating control amplifier(29, 30) connected to said sensing means (1) and providing an outputsignal (29'') which is available to command the relative mass ratio (lambda ) of airfuel applied to the engine; and a threshold switch (15),connecting the sensing means (1) to the integrating control amplifierand operative to change switching state above, and below its thresholdlevel, said threshold switch, upon normal operation of the system,cyclically switching between its states, a timing circuit (36) connectedto the threshold switch (15) and measuring the time intervals betweenchange-over of switching state of the threshold switch (15), asdetermined by changing output from the sensing means (1) the timingcircuit sensing the intervals between change-over of the thresholdswitch (15''); and a change-over switch (46; 47, 49) controlled by saidtiming circuit (36) the timing circuit controlling said change-overswitch to be in a first state when the duration of the intervals betweenchange-over of the threshold switch are below a predetermined timinginterval, indicative of proper operation of the sensing means and of thethreshold switch, the timing circuit controlling said change-over switch(46; 47, 49) to be in another state when the switching interval sincethe last change-over of the threshold switch is above said predeterminedinterval, said change-over switch, when in the second state, providingan output signal (49'') indicative of excess time lapse betweenchange-over of the threshold switch (15) and representative of systemmalfunction.
 2. System according to claim 1, further comprising amonostable flip-flop (37, 39) connected between the Threshold switch(15) and the timing circuit (36) and triggered into unstable state bychange of state of the threshold switch (15); and wherein said timingcircuit comprises a capacitor (44) connected to be charged during onestate of the monostable flip-flop (37, 39) and a discharge circuit (45)permitting discharge of the capacitor during the other state of themonostable flip-flop, said discharge circuit having a time constantwhich prevents complete discharge of the capacitor when the switchinginterval of the threshold switch (15) is within said predetermined valueto provide a residual charge on the capacitor (44), said residual chargeholding said change-over switch (46) in said state indicative of properoperation.
 3. System according to claim 2, wherein the capacitor (44) ischarged during the unstable state of the monostable flip-flop (37, 39)and discharged during the stable state thereof and said dischargecircuit has a high time constant and is connected to and controllingsaid change-over switch (46) to change state indicative of''''trouble'''' operation upon discharge of the capacitor below apredetermined level.
 4. System according to claim 1, wherein theintegrating control amplifier comprises an operational amplifier (29);and a coupling circuit (54) is provided between the output (49'') of thechange-over switch (46) and the inverting input of the operationalamplifier (29) to modify the input signal applied thereto.
 5. Systemaccording to claim 4, wherein the coupling circuit comprises a diode(54).
 6. System according to claim 4, further comprising selectivelyvariable input resistance means (28, 31, 32) connected to the couplingnetwork and the operational amplifier (29, 30).
 7. System according toclaim 2, wherein the monostable flip-flop (37, 39) comprises atransistor (37) having its emitter-collector path connected in parallelto the capacitor (44) of the timing circuit; and a coupling capacitor(39) and a diode (38) coupling network connecting the output of thethreshold switch (15) to the control electrode of the transistor (37).8. System according to claim 1, wherein the timing circuit comprises acapacitor (44); and the change-over switch (46) comprises anemitter-follower input transistor (47) connected to the timing capacitor(44), and a switching output transistor (49) connected to and controlledby said input transistor (47).
 9. System according to claim 1, furthercomprising alarm circuit means (53'', 60, 61) connected to the output(49'') of the change-over switch (46) to provide an alarm output signalupon sensing of ''''trouble'''' operation.
 10. System according to claim1, further comprising at least one override connection terminal (52,52'') connected to the change-over switch (46) to hold said switch in aposition indicating proper operation of the sensing means to overridecontrol of the integrating control amplifier (29, 30) by saidchange-over switch (46) upon energization of at least one of saidoverride terminals.
 11. System according to claim 1, wherein therelative mass ratio ( lambda ) of the air-fuel applied to the engine iscontrolled to apply a lean mixture to the engine when the timing circuithas sensed a time interval from the last change-over of the thresholdswitch (15) which is above the time duration of the predeterminedinterval.